WO1993025924A1

WO1993025924A1 - Optical polymer element for coupling photoelements to integrated optical circuits

Info

Publication number: WO1993025924A1
Application number: PCT/DE1993/000475
Authority: WO
Inventors: Klaus-Michael Mayer
Original assignee: Robert Bosch Gmbh
Priority date: 1992-06-15
Filing date: 1993-06-03
Publication date: 1993-12-23
Also published as: JPH07508596A; KR950702039A; EP0647328A1; EP0647328B1; DE59309260D1; US5513288A; DE4220135A1

Abstract

An optical polymer element for coupling photoelements to integrated optical circuits is disclosed, in which only the evanescent field components of an optical waveguide are coupled into the photoelements. A coupling element, preferably a buffering layer (30), is arranged between a light waveguide (20) and the photoelement (23). The buffering layer (30) has in the area of the photoelement (23) an index of refraction that is less than or equal to the index of refraction of the light waveguide (20), but that is greater than the index of refraction of the buffering layer (30) outside of the area of the photoelement (23).

Description

Optical polymer element for coupling photo elements to integrated optical circuits

The invention relates to an optical polymer element according to the preamble of the main claim and is preferably used for coupling suitable photodiodes to components of integrated optics.

The increasing use of integrated optical components for optical communications technology, for sensors and for the computer sector means that optical connection technology is becoming increasingly important.

Components of the integrated optics (10) for optical communications technology (wavelength range 1300 to 1550 nm) as well as for optical sensors (usually in the wavelength range 633-850 nm) require opto-electronic signal conversion at the interface between optical and electronic signal processing. This is done, for example, by coupling the signal light into a photodiode with a corresponding spectral sensitivity (for example, for the optical communication technology InP-Co pounds, for the sensor system Si photo diodes). The usual way of coupling a photodiode to an optical fiber is to directly couple the photodiode to the end of the waveguide ("butt coupling"). The light energy is completely conducted into the diode and converted into electronic excitation states there. In addition, an optical waveguide can also be weakly coupled to a photodiode by merely coupling its evanescent field components into the diode ("leaky wave coupling") - the size of the electronic signal response is a function of coupling strength and coupling length. Alternatively, an optical waveguide can also be passed through an optical semiconductor amplifier (essentially a semiconductor laser diode with anti-reflective end faces) and the degradation of the charge carrier inversion can be tapped off as an electronic signal via the external power supply of the amplifier diode.

The known components have the disadvantage that they are only relatively expensive to manufacture.

Advantages of the invention

The optical polymer element according to the invention, according to the characterizing part of the main claim, offers the advantage over that the simplest coupling and fastening of photo elements to integrated optical polymer optical waveguides is possible, the polymer element is compatible with planarly integrated electronics and large cost advantages are achieved through mass production. For this purpose, a coupling element, preferably a buffer layer, is provided between the optical waveguide and the photo element, the buffer layer in the area of the photo element having a refractive index which is less than or equal to the refractive index of the optical waveguide, but greater than the refractive index of the buffer layer outside the area of the photo element.

Further advantageous refinements are specified in the subclaims.

A preferred solution is if the photodiodes are integrated directly and planarly and monolithically into suitable substrates (for example silicon) by means of diffusion processes and / or ion implantation. The electrical wiring is advantageously carried out directly on the chip. In order to apply optical light waveguides to such an electronically processed chip, an optical buffer layer with a lower refractive index than the light-guiding layer is first applied. This is followed by the light-conducting polymer layer with the laterally structured optical waveguides and, optionally, an upper covering layer with a lower refractive index. If the buffer layer is now optically thin (thickness <1 / e drop in the field distribution), the evanescent field components reach into the substrate and lead to severe loss of intensity. Conversely, if the buffer layer is optically thick (thickness >> 1 / e drop in the field distribution), no light is coupled into the photodiode in the area either. The necessary high and local coupling of the optical waveguide to the photodiode is now achieved in that the "optically isolating" buffer layer can be optically changed locally over the sensitive window of the photodiode so that the evanescent fields locally far over the buffer layer can protrude (and into the photodiode). For this purpose, the buffer layer can be masked such that local ion diffusion or ion implantation only raises the refractive index in a desired window above the photodiode. The mask is removed after the process and the optical waveguide and cover layer are applied. The degree of coupling of the optical waveguide can be set via the index change in the buffer layer.

An advantageous possibility of implementation is provided by a polymer layer (for example PMMA as an optical buffer) with photopolymerizable additives (for example benzil dimethyl ketal). Here the refractive index of the buffer layer can be raised locally above the photodiode by simple UV exposure using mask technology. The index can also be gradually changed spatially by UV exposure of structures of varying density (gray wedge) if index jumps and possibly resulting disturbances in the monomode of the waveguide are to be avoided. Depending on the photopolymerization behavior of the optical buffer, either an index profile of the buffer in the longitudinal direction of the waveguide can be generated (ie a slightly higher refractive index at the edges of the photodiode, stronger) increased index above the detection window of the diode) or a spatial depth profile of the index increase in the buffer layer (ie superficial index increase at the diode edges, deeper index increase above the detection window of the diode =>"verticaltaper").

In both borderline cases (mixtures are possible), the refractive index, which is slightly raised above the photodiode, results in weaker guidance of the light, which is equivalent to a widened field distribution in the direction of the diode. The adiabatic index changes (eg gray wedge exposures) keep the disturbance of the guiding properties sufficiently small to preserve the monomode of the waveguide - this is important if the signal is looped past the photodiode (and is only weakened by the detection) ) or if the signal is to be coupled into wavelengths in optical resonators selectively without having a massive influence on the mode distribution in the resonator.

The buffer layer can also itself be laterally photostructured by exposing a tapered taper structure with a slightly increased index into it. In this case too, the index manipulation of the buffer layer gradually extends the light field of an overlying waveguide (by adiabatic activation of the disturbance) towards the diode. If the refractive index of the buffer layer in the tapered area is set even higher than that of the optical waveguide lying above it, the light field is completely removed from the Waveguide are withdrawn and fed to the diode.

After the index manipulation of the buffer layer, as described above, the optical waveguide is structured in the layer above. What is essential here is the basic possibility of subsequently overlaying any preprocessed and wired electronic chips with a polymeric “optical connection level”, which can be done without disturbing the electronic components due to the low process temperatures required for producing polymer optical fibers.

It is also in accordance with the invention if optical waveguides in the form of small, precision-made trenches are predefined on a "master structure" and are further processed by means of an electroplating molding for use in molds (tools) for injection molding or injection molding processes. This enables mass-production duplication of optical fiber optic components. The small trenches in the embossed "daughter structures" (polymer substrates) are filled with a higher refractive index optical polymer (channel waveguide) and a polymeric (low refractive) cover plate (superstrate) is closed at the top to complete a fully polymer component. The cover plate can, for example, be made of the same material as the waveguide daughter structures and then define a passive component. The cover plate can also be used hybrid carry electronic components (eg photodiodes) which can then be optically coupled to the optical waveguide. For this purpose, a suitable master structure with receiving openings for the electronic components is produced for the cover plate. This can be done by anisotropic precision etching of recesses in silicon wafers or by so-called X-ray depth lithography in PMMA materials. Like the optical waveguides, the cover structures can be electroplated from this and reproduced in injection molding / injection molding processes.

The photodiodes (and possibly further components) are inserted and fastened individually (or preferably as a detector array) into the depressions in the cover plate. Before the cover plate and base plate prepared in this way fit together with the optical light waveguides (in the form of small trenches with filled, higher refractive index liquid polymer), an optical buffer layer must be worked in between the light waveguides and the electronics in the cover plate, which via optical coupling the evaniscent field components are only allowed in the area of the diode entry window.

There are two advantageous ways of doing this:

a) The electrical wiring of the electronics is carried out by depositing corresponding conductor tracks on the populated cover plate. Beyond that Polymer layer (index like cover plate) with photo-polymerizable admixtures applied as an optical buffer. By means of local UV exposure (mask technique), the index in the region of the photodiode, as described above, can be set with or without taper structures so that here - and only here - the light coupling is possible in the fully assembled component.

b) The electrical conductor tracks are produced on one side of a thin polymer film. This must be suitable as an optical buffer layer, i.e. low attenuation and a refractive index less than that of the waveguide (optical film thickness >> 1 / e drop in the optical fields). The film could, for example, consist of the same material from which daughter structures and lid structures are also made. For thicknesses in the μm range, this film is preferably first stabilized by a carrier film.

The buffer film is laminated onto the cover plate with the electrical conductor tracks in order to ensure contacting of the electronic assemblies (the backing film on the back can then be removed).

The refractive index of the buffer film can now be locally raised again in the region of the diode window by diffusion or implantation processes or by UV exposures (with correspondingly crosslinkable oligomers in the film), and the degree of coupling can thus be set. Again the different possibilities of lateral and vertical taping are available. However, a thermoplastic film can also be hot-stamped in the region of the diode window in an advantageous manner by means of a correspondingly shaped stamping tool. This results in a locally slightly thinner layer thickness of the optical buffer defined by the embossing tools and ultimately a stronger field coupling in the desired areas. The field distributions are in turn comparable with the qualitative profiles already mentioned.

Finally, the complete component is assembled with a precise fit, so that the photodiode windows come to lie exactly over the associated optical fibers. The higher refractive index polymer in the light guide trenches can be designed as a thermally or UV-crosslinking adhesive and thus ensure the mechanical connection of the assemblies. At the same time, this liquid polymer compensates for any differences in thickness from the cover plate, for example in the case of embossed buffer films. With its applied conductor tracks, the buffer film can protrude laterally beyond the cover plate and allow simple electrical contacting to the outside.

It is also within the meaning of the invention if an optical waveguide, which has been produced in an organic polymer film, for example by local photopolymerization, by so-called "UV bleaching" or by another structuring technique, by an optically higher-refractive, transparent adhesive (for example UV polymerizing adhesive) coupled to the photodiode becomes. Its photosensitive side is located directly above the optical waveguide which is not covered at the top. The degree of optical coupling can be set by selecting the refractive index, the thickness and the area (length in the waveguide direction) of the polymer adhesive. Depending on the degree of coupling, the light can be decoupled from the optical waveguide (and coupled into the photodiode in the loss-free limit case), or can only be weakened slightly, for example to tap signals from a data bus without affecting its optical transparency. A suitable coupling distance can be set by the shape of the diode. The optical adhesive serves both for optical coupling and for mechanical fixing. The entire chip can subsequently be encapsulated and protected with a low-refractive cover layer.

Examples of applications include integrated optical displacement / angle sensors or communications technology receiver stations, which can thus be installed in a cost-effective manner. In the case of a wavelength-selective detection, the signals would have to be filtered out by wavelength-selective couplers or integrated optical resonators on the optical chip and fed to the photo receiver.

The invention will be explained in more detail below in exemplary embodiments with reference to the associated drawings. Show it: Figure 1 is a perspective view of the basic coupling of a photodiode;

FIG. 2 shows a section through the coupling point according to FIG. 1;

Figure 3 shows a coupling point on another example in plan view;

FIG. 4 shows a section through the coupling point according to FIG. 3;

5 shows a coupling point according to FIG. 3 with a tapered buffer area;

FIG. 6 shows a section through the coupling point according to FIG. 5;

FIG. 7 shows a perspective view of a polymer waveguide component before assembly;

FIG. 8 shows a section through the polymer waveguide component according to FIG. 7 in the assembled state;

FIG. 9 shows a perspective view of a polymer waveguide component with an integrated photodiode before assembly;

FIG. 10 shows a section through the polymer waveguide component according to FIG. 9 in the assembled state; FIG. 11 shows a perspective view of a further polymer waveguide component with an integrated photo diode before assembly;

FIG. 12 shows a section through the polymer waveguide component according to FIG. 11 in the assembled state;

FIG. 13 shows a section through a further polymer waveguide component with an integrated photodiode and

FIG. 14 shows a section through a further polymer waveguide component with an integrated photodiode.

In all the figures, the same parts, also the same parts, are denoted by the same reference symbols.

FIG. 1 shows an optical waveguide 20 which is arranged in a polymer film 22 provided on a substrate plate 21. A photodiode 23 is glued onto the optical waveguide 20 by means of a polymer adhesive 24.

It becomes clearer in FIG. 2 that the photodiode 23 is at a certain distance from the polymer film 22 and thus from the optical waveguide 20. This distance is determined by the spacers 25. A buffer layer 26 is provided between the substrate plate 21 and the polymer film 22. The photodiode 23 is directly over with its photosensitive side the optical fiber 20 arranged. The entire component is cast with a cover layer 27. The following relationships apply to the refractive indices of the individual components:

The refractive index ng of the cover layer 27 and the refractive index n4 of the buffer layer 26 is less than or equal to the refractive index n _{3 of} the polymer film 22. The refractive index n ^ of the optical waveguide 20 is greater than n ₃ . The refractive index

of

Polymer adhesive 24 is less than or equal to the refractive index n ^ of the optical waveguide 20.

The principle of operation is as follows:

A light pulse conducted through the optical waveguide 20 is decoupled from the optical waveguide 20 in accordance with the set refractive index and the thickness and length of the polymer adhesive 24, depending on the application, partially or completely, and fed to the photodiode, which performs its predetermined function there. The spacers 25 can be applied to the photodiode 23 and can, for example, be galvanically reinforced metal surfaces to the desired thickness (= distance), which at the same time serve as conductor tracks for continuing the signal of the photodiode 23.

The example shown in FIGS. 3 and 4 consists of a photodiode 23 integrated in a substrate plate 21, via which the optical waveguide 20 runs separately through a buffer layer 30. The buffer layer 30 has a region 31 which is directly between the optical waveguide 20 and the photosensitive window of the photodiode 23 is arranged.

The following relationships apply to the refractive indices:

The refractive index TΪ _{Q of} the cover layer 27 is smaller than or equal to the refractive index n _{3 of} the buffer layer 30, which in turn is smaller than the refractive index n _{2 of} the region 31, which is smaller than the refractive index n ^ of the optical waveguide 20. The qualitative field distribution outside the region 31 is shown in FIG. The light pulse is guided within the optical waveguide 20. The qualitative field distribution in the region 31, here designated 33, makes it clear that the higher refractive index of the buffer layer 30 means that the evanescent field components locally extend far beyond the buffer layer 30 and thus protrude into the photosensitive window of the photodiode 23. The intended detection function of the photodiode 23 is thus triggered.

FIGS. 5 and 6 show in a preferred embodiment with an analog structure according to FIGS. 3 and 4 how area 31 is tapered. A vertical taper is shown schematically on the left side in 40 and a lateral taper is shown schematically on the right side in 41 (either the one or the other taper shape is then used on both diode sides in specific components).

In the area 40, the taper has the on the edges Photodiode 23 has a slightly increased refractive index and a more elevated refractive index above the photosensitive window of the photo diode 23. The course of the refractive index increase is indicated by line 42. However, the taper can also have a laterally tapered course, as shown in region 41. Here the refractive index decreases in the lateral direction until the taper runs out outside the photodiode 23. What has already been said about FIGS. 3 and 4 applies to the relationships of the refractive indices and the qualitative field distribution.

The further examples relate to fully polymeric components which are produced using impression technology.

The basic structure of fully polymeric components is illustrated in FIGS. 7 and 8. Highly refractive optical polymers, which form the optical waveguide 20, are cast into a base plate 50 made of polymer substrate in precision-made sizes. The base plate 50 is covered with a cover plate 51, which can consist of the same polymer substrate as the base plate 50. The connection is made by means of a liquid polymer 52, which can be identical to the polymer of the optical waveguide 20.

As shown in FIGS. 9 and 10, a photodiode 23 is inserted into a depression 53 in the cover plate 51. An optical buffer layer 54 is arranged between the base plate 50 or the adhesive 52 and the cover plate 51. The optical buffer layer 54 has an area 55 which is only in this area permits coupling between the optical waveguide 20 and the photodiode 23. The area 55 can in turn be formed like the area 31 described in FIGS. 3 to 6, that is to say without or with taper structures. The electrical connecting tracks 56 of the photodiode 23 are guided outwards on the equipped cover plate 51.

The indices of refraction of the individual regions behave analogously to those described in FIGS. 1 to 4, the indices of refraction of substrate plate 50 and cover plate 51 being less than or equal to the refractive index of the buffer layer 54.

A light pulse coming through the optical waveguide 20 acts with its evanescent field components in the area of the photoactive window of the photodiode 23, and only here, through the buffer layer 54 and triggers the desired switching function in the photodiode 23, which via the connecting tracks 56 can be tapped.

Another example is shown in FIGS. 11 and 12. The electrical connection tracks 56 of the photodiode 23 are applied to a thin polymer film which also serves as an optical buffer layer 54. The film is laminated on the cover plate 51 with a precise fit before assembly. All other components and functions have already been described in detail for the other examples.

The example according to FIG. 13 has an optical buffer layer 54 in the form of a film, as described in FIGS. 11 and 12, which is in the region of the photoactive window. The embossing is carried out in such a way that an area 60 results which produces a locally defined, slightly thinner layer thickness of the buffer layer 54. By setting this layer thickness, the coupling of the photodiode 23 to the optical waveguide 20 is set in the manner already described.

In a further embodiment, as shown in FIG. 14, in addition to the described index manipulation of the buffer layer 54, which makes it “optically thinner” and thus consistently designed for the light waves, the light entry surface of the photodiode 23 can also be brought closer geometrically to the optical waveguide. For this purpose, the photodiode structure, an InP-based technology is described in the example without restricting generality, can be selectively overgrown with an InP cover layer 70 over the light entry window, which can typically be 0.2 to 1 μm thick. If a planarizing polymer buffer 54 and the optical waveguide 20 are applied over such a diode structure, the buffer effect in the area of the diode entry window is significantly reduced and because of the higher refractive index of the semiconductor materials (typically n 3.5) the evanescent light pulled out of the optical waveguide and fed to the light-sensitive p / n junction of the semiconductor diode for detection.

This geometric effect of the photodiode, which projects into the buffer layer, can additionally be combined with photopolymerized, tapered structures in the buffer layer in the sense already described.

Claims

- 1.Patent claims

1. Optical polymer element for coupling photo elements to integrated optical circuits, only the evanescent field components of an optical waveguide being coupled into the photo elements, characterized in that a coupling element, preferably, between an optical waveguide (20) and the photo element (23) a buffer layer is arranged, the buffer layer having a refractive index in the region of the photo element (23) which is less than or equal to the refractive index of the optical waveguide (20) but larger than the refractive index of the buffer layer outside the region of the photo element (23 ) is.

2. Optical polymer element according to claim 1, characterized in that the photo elements (23) are directly planar and monolithically integrated into suitable substrates by diffusion processes and / or ion implantation, and that the substrate and the photo elements (23) are a buffer layer (30 ) on which a light-guiding, laterally structured optical waveguide polymer layer is applied.

3. Optical polymer element according to claim 2, characterized in that the buffer layer (30) locally over the sensitive window of the photo element (23) is optically changed so that the evanescent light fields locally far beyond the buffer layer and into the photo element (23) protrude.

4. Optical polymer element according to one of the preceding claims, characterized in that the buffer layer (30) has a lower overall refractive index than the light-guiding polymer layer.

5. Optical polymer element according to one of the preceding claims, characterized in that the refractive index of the buffer layer (30) is raised by local ion diffusion or ion implantation in the region of the sensitive window of the photo element (23).

6. Optical polymer element according to one of the preceding claims, characterized in that the degree of optical coupling is set via the level of the index change of the buffer layer (30).

7. Optical polymer element according to one of the preceding claims, characterized in that a polymer layer with photo-polymerisable additives is used as the buffer layer (30), the refractive index of which is set locally above the sensitive window of the photo element (23) by UV exposure is. - έo -

8. Optical polymer element according to one of the preceding claims, characterized in that the refractive index is spatially gradually changed by UV exposure in a varied density and thus the refractive index profile of the buffer layer in the waveguide longitudinal direction and / or alε spatial depth profile of the refractive index increase above the sensitive window deε Photoelementeε is set changed.

9. Optical polymer element according to one of the preceding claims, characterized in that the buffer layer (30) is photo-structured in the lateral direction by exposing a taper structure (40; 41) with a slightly increased index.

10. Optiεcheε polymer element according to one of the preceding claims, characterized in that the taper structure is tapered.

11. Optical polymer element according to claim 1, characterized in that the photo elements (23) are introduced into a polymeric top plate (51) which is closed at the top, the top plate (51) fits precisely onto a base plate (50) having an optical waveguide (20) ) is added and an optical buffer layer (54) is introduced between the cover plate (51) and the base plate (50).

12. Optical polymer element according to claim 11, characterized in that the optical buffer layer (54) permits optical coupling to the optical waveguide (20) only in the area of the sensitive window of the photo element (23).

13. Optical polymer element according to one of the preceding claims, characterized in that the optical waveguides (20) consist of higher-refractive polymers introduced in a polymeric base plate into structures formed as trenches.

14. Optical polymer element according to one of the preceding claims, characterized in that the receiving openings (53) for the photo elements (23) in the cover plate (51) by setting a master structure which is electroplated and reproduced in injection molding / injection molding processes , are manufactured.

15. Optical polymer element according to one of the preceding claims, characterized in that the base plate (50) and the cover plate (51) can be produced simultaneously.

16. Optical "polymer element according to one of the preceding claims, characterized in that the optical buffer layer (54) is applied to the cover plate (51) with photopolymerizable admixtures and by local UV exposure in the region of the sensitive window of the photoelectric element tes (23) the refractive index in the waveguide longitudinal direction and / or as a spatial depth profile of the refractive index increase and / or as a taper structure can be set.

17. Optical polymer element according to one of the preceding claims, characterized in that the optical buffer layer (54) as a thin polymer film is formed, on the one side of the cover plate (51) facing electrical conductor tracks (56) are arranged and this buffer film is laminated with the electrical conductor tracks on the cover plate (51) and thus at the same time electrically contacts the photo elements (23).

18. Optical polymer element according to one of the preceding claims, characterized in that the refractive index of the buffer film (54) in the region of the sensitive window of the photo element (23) can be raised locally by diffusion or implantation processes or UV exposure.

19. Optical polymer element according to one of the preceding claims, characterized in that the degree of optical coupling can be set via the refractive index.

20. Optical polymer element according to one of the preceding claims, characterized in that the refractive index can be varied widely and laterally and / or vertically.

21. Optical polymer element according to one of the preceding claims, characterized in that the optical buffer layer (54) consists of a thermoplastic film embossed by means of an embossing tool.

22. Optical polymer element according to one of the preceding claims, characterized in that the optical buffer layer (54) locally has a smaller layer thickness in the region (60) of the sensitive window of the photo element (23).

23. Optical polymer element according to one of the preceding claims, characterized in that the optical conductor tracks (56) leading optical buffer layer (54) is led beyond the outer dimensions of the components and has an electrical contact there.

24. Optical polymer element according to claim 1, characterized in that the light entry surface of the photo element (23) is geometrically brought closer to the optical waveguide (20) and partially projects into the buffer layer (54).

25. Optical polymer element according to claim 24, characterized by _/ that the light entry surface is covered with a semiconductor layer (70), the semiconductor layer (70) has a higher refractive index than the optical waveguide (20), and the evaneεzent light fields pull out the optical waveguide and feeds the light-sensitive transition of the semiconductor for detection.

26. Optical polymer element according to claim 1, characterized in that an optical waveguide (20) is produced in an organic polymer film (22) and the photosensitive side of the photo element (23) with a higher refractive index transparent adhesive (24) directly on the Licht¬ waveguide (20) is glued.

27. Optical polymer element according to one of the preceding claims, characterized in that the degree of optical coupling can be set by selecting the refractive index, the thickness and the length of the polymer adhesive. - 2k -

28. Optical polymer element according to one of the preceding claims, characterized in that the coupling distance can be adjusted via the spacer (25) arranged between the polymer (22) receiving the optical waveguide (20) and the photoelement (23).

29. Optical polymer element according to one of the preceding claims, characterized in that the spacers (25) are formed by electroplated conductor tracks applied to the photoelement (23).

30. Optical polymer element according to one of the preceding claims, characterized in that the coupling point simultaneously realizes the mechanical fixation of the photoelement (23).

31. Optical polymer element according to one of the preceding claims, characterized in that the optical waveguide (20) is potted together with the coupled photo element (23) with a low-refractive cover layer (27).